Norepinephrine overflow in perfused mesenteric arteries of spontaneously hypertensive rats

Enhancement of muscarinic receptor-mediated excitation in spontaneously hypertensive rat adrenal medullary chromaffin cells

One of the mechanisms for hypertension is an increase in blood catecholamines due to increased secretion from sympathetic nerve terminals and adrenal medullary chromaffin (AMC) cells. Spontaneously hypertensive rats (SHRs) are used as an animal model of hypertension. Catecholamine secretion in AMC cells occurs in response to humoral factors and neuronal inputs from the sympathetic nerve fibres. Acetylcholine (ACh) released from the nerve terminals activates nicotinic as well as muscarinic ACh receptors. The present experiment aimed to elucidate whether muscarinic receptor-mediated excitation is altered in SHR AMC cells and, if it is, how. Compared with normotensive rat AMC cells, muscarinic stimulation induced greater catecholamine secretion and larger depolarising inward currents in SHR AMC cells. In contrast to normotensive rat AMC cells, the muscarine-induced current consisted of quinine-sensitive and quinine-insensitive components. The former and the latter are possibly ascribed to nonselective cation channel activation and TWIK-related acid-sensitive K+ (TASK) channel inhibition, as noted in guinea pig AMC cells. In fact, immunoreactive material for TASK1 and several isoforms of transient receptor potential canonical (TRPC) channels was detected in SHR AMC cells. Stromal interaction molecule 1 (STIM1), which plays an essential role for heteromeric TRPC1-TRPC4 channel formation and is not expressed in normotensive rat AMC cells, was detected in the cytoplasm and co-localised with TRPC1. The expression of muscarinic M1 receptors was enhanced in SHR AMC cells compared with normotensive rats. The results indicate that muscarinic excitation is enhanced in SHR AMC cells, probably through facilitation of TRPC channel signalling.

The abnormalities of adrenomedullary hormonal system in genetic hypertension: Their contribution to altered regulation of blood pressure

It is widely accepted that sympathetic nervous system plays a crucial role in the development of hypertension. On the other hand, the role of adrenal medulla (the adrenomedullary component of the sympathoadrenal system) in the development and maintenance of high blood pressure in man as well as in experimental models of hypertension is still controversial. Spontaneously hypertensive rats (SHR) are the most widely used animal model of human essential hypertension characterized by sympathetic hyperactivity. However, the persistence of moderately elevated blood pressure in SHR subjected to sympathectomy neonatally as well as the resistance of adult SHR to the treatment by sympatholytic drugs suggests that other factors (including enhanced activity of the adrenomedullary hormonal system) are involved in the pathogenesis of hypertension of SHR. This review describes abnormalities in adrenomedullary hormonal system of SHR rats starting with the hyperactivity of brain centers regulating sympathetic outflow, through the exaggerated activation of sympathoadrenal preganglionic neurons, to the local changes in chromaffin cells of adrenal medulla. All the above alterations might contribute to the enhanced release of epinephrine and/or norepinephrine from adrenal medulla. Special attention is paid to the alterations in the expression of genes involved in catecholamine biosynthesis, storage, release, reuptake, degradation and adrenergic receptors in chromaffin cells of SHR. The contribution of the adrenomedullary hormonal system to the development and maintenance of hypertension as well as its importance during stressful conditions is also discussed.

Effects of nicorandil on sympathetic neurotransmission in rat caudal artery

1. We examined the effects of nicorandil, an ATP-sensitive potassium (K(ATP)) channel opener and nitric oxide donor, on the release of noradrenaline from vascular sympathetic nerves. This effect was compared to the effect on vascular smooth muscle. 2. Caudal artery preparations from Wistar rats were electrically stimulated (1 Hz, 0.5-ms duration) and noradrenaline release in the artery was detected using an high-pressure liquid chromatography-electrochemical detection technique. The pharmacological properties of the prejunctional effect of nicorandil were determined using the nonselective K(ATP) channel blocker glibenclamide, the pancreatic beta-cell and brain-type K(ATP) channel blocker tolbutamide, and the smooth muscle-type K(ATP) channel blocker PNU-37883A. 3. Nicorandil inhibited the electrical stimulation-evoked noradrenaline release in a concentration-dependent manner. This inhibitory effect was abolished by 1 micromol/L glibenclamide and 10 micromol/L tolbutamide, but was not affected by 10 micromol/L PNU-37883A or 0.3 micromol/L ODQ. Nicorandil did not affect the noradrenaline transporter uptake 1 in the adrenergic nerve terminals. 4. Nicorandil produced a relaxation response in a concentration-dependent manner in the caudal artery pre-contracted with 0.3 micromol/L noradrenaline. This relaxation response was significantly diminished in the presence of 1 micromol/L glibenclamide, 10 micromol/L PNU-37883A and 0.3 micromol/L ODQ but not by 10 micromol/L tolbutamide. 5. These findings suggest that nicorandil inhibits noradrenaline release via the K(ATP) channels of sympathetic nerves. These channels may be pharmacologically different from those of vascular smooth muscle.

Role of cholinergic receptors in adrenal catecholamine secretion in spontaneously hypertensive rats

We investigated the role of nicotinic and muscarinic receptors in secretion of catecholamines induced by transmural electrical stimulation (ES) from isolated perfused adrenal glands of spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats. ES (1-10 Hz) produced frequency-dependent increases in epinephrine (Epi) and norepinephrine (NE) output as measured in perfusate. The ES-induced increases in NE output, but not Epi output, were significantly greater in adrenal glands of SHRs than in those of WKY rats. Hexamethonium (10-100 microM) markedly inhibited the ES-induced increases in Epi and NE output from adrenal glands of SHRs and WKY rats. Atropine (0.3-3 microM) inhibited the ES-induced increases in Epi and NE output from adrenal glands of SHRs, but not from those of WKY rats. These results suggest that endogenous acetylcholine-induced secretion of adrenal catecholamines is predominantly mediated by nicotinic receptors in SHRs and WKY rats and that the contribution of muscarinic receptors may be different between these two strains.

Impaired function of alpha-2 adrenoceptors in smooth muscle of mesenteric arteries from spontaneously hypertensive rats

The alpha2-adrenoceptor function in mesenteric arteries of spontaneously hypertensive rats (SHR) was investigated by comparing membrane potential changes in response to adrenergic agonists in preparations from female SHR, Wistar-Kyoto (WKY) and normotensive Wistar rats (NWR). Resting membrane potential was found to be less negative in mesenteric arteries from SHR than in those from NWR and WKY. Apamin induced a decrease in the membrane potential of mesenteric artery rings without endothelium from NWR and WKY, but had no effects in those from SHR. Both UK 14,304 and adrenaline, in the presence of prazosin, induced a hyperpolarization that was significantly lower in de-endothelialized mesenteric rings from SHR than in those from NWR and WKY. In mesenteric rings with endothelium, however, similar hyperpolarization was observed in the three strains. In NWR mesenteric rings with endothelium the hyperpolarization induced by activation of alpha2-adrenoceptors was abolished by apamin, whereas in intact SHR mesenteric rings this hyperpolarization was slightly reduced by apamin and more efficiently reduced by Nomega-nitro-L-arginine. It is concluded that the activity of potassium channels coupled to alpha2-adrenoceptors is altered in the smooth muscle cells of SHR mesenteric arteries, contributing to their less negative membrane potential. On the other hand, the endothelial alpha2-receptors are functioning in mesenteric vessels from SHR and their stimulation induces a hyperpolarization mainly through the release of nitric oxide.

Influence of adrenodemedullation on beta 2- and beta 3-adrenoceptors mediating relaxation of oesophageal smooth muscle of spontaneously hypertensive rats

1. In oesophageal smooth muscle strips from spontaneously hypertensive rats (SHR) of 8-10 and 22-24 weeks of age, respectively, beta-adrenoceptor-mediated relaxation was investigated, by use of the beta-agonists, (-)-isoprenaline and fenoterol (both in the absence and presence of the beta 2-selective antagonist ICI 118,551) and the selective beta 3-agonist, BRL 37,344. 2. In preparations from 8-10 week SHR, (-)-isoprenaline- and fenoterol-induced concentration-response curves (CRCs) were hardly antagonized by ICI 118,551 at concentrations up to 1 microM, indicating only a minor contribution of beta 2-adrenoceptors. pA2-values for ICI 118,551 of 5.30 ((-)-isoprenaline as agonist) and 5.46 (fenoterol as agonist), estimated from the shifts at the highest (10-100 microM) antagonist concentrations, are consistent with affinity at a beta 3-adrenoceptor, similar to that in Wistar rat oesophageal smooth muscle. 3. In 8-10 week SHR, adrenodemedullated at 4 weeks of age (SHR-ADM4) the potency of fenoterol was markedly increased and CRCs were shallow. In addition, ICI 118,551 (0.1 microM) now produced a clear rightward shift accompanied by a steepening of the CRC. A marked further shift was observed only at 100 microM of the antagonist. The data are compatible with the involvement of both beta 2- and beta 3-adrenoceptors. 4. In 22-24 week animals, the same differences between SHR and SHR-ADM4 were observed with fenoterol as in 8-10 week animals, though beta-adrenoceptor responsiveness was slightly decreased. The potency of ICI 118,551 at beta 3-adrenoceptors (pA2 = 5.11) was significantly different from the pA2 value of 5.46 obtained with the younger animals. 5. Responses to the beta 3-adrenoceptor agonist, BRL 37,344, were similar in Wistar rat and SHR preparations. In 8-10 week SHR, a small decrease in the maximal response was observed, which in animals of 22-24 weeks of age was accompanied by a small decrease in the pEC50 value as well. 6. The results clearly indicate that beta 2-adrenoceptors in SHR oesophageal muscularis mucosae are desensitized, whereas beta 3-adrenoceptor-mediated responses are unaffected and similar to the responses observed in the Wistar rat oesophagus. The functional presence of beta 2-adrenoceptor-responses in SHR-ADM4 suggests a major role for adrenal-derived adrenaline in the desensitization of the beta 2-adrenoceptor-population.

In vitro perfusion studies of human resistance artery function in essential hypertension

To simulate in vivo conditions as closely as possible to in vitro conditions, we examined the morphological and functional characteristics of isolated human subcutaneous small arteries from 17 essential hypertensive patients and 14 normotensive control subjects using a perfusion myograph. Vessel segments were cannulated and exposed to conditions of constant flow and pressure. The ratio of media thickness to lumen diameter in arteries from hypertensive patients increased significantly. With the endothelium intact, sensitivity to extraluminally applied norepinephrine was not different, and this was not affected by inhibition of neuronal amine uptake with cocaine. After removal of the endothelium, sensitivity to norepinephrine was augmented in normotensive vessels to a greater extent than in hypertensive vessels. Endothelium-dependent relaxation to acetylcholine was significantly reduced in arteries from hypertensive patients, but endothelium-independent relaxation to sodium nitroprusside was not different from that observed in vessels from normotensive control subjects. These data demonstrate that sensitivity to exogenous norepinephrine is not different in essential hypertension but that there is defective endothelium-dependent dilatation, suggesting a contributory role for endothelium dysfunction in human essential hypertension.

Sympathetic hyperinnervation protects vascular smooth muscle cells from necrosis in stroke-prone spontaneously hypertensive rats

Sympathetic nerve fiber distribution and vascular smooth muscle morphology were investigated in the ophthalmic artery of stroke-prone spontaneously hypertensive rats (SHRSP) and were compared with those of normotensive Wistar Kyoto (WKY) rats at the age of 120 days. The distribution of fluorescent noradrenergic (NA) nerve fibers was examined by the glyoxylic acid method. The ophthalmic artery was divided into two portions according to the size of the outer diameter, that is into a proximal portion (above 100 microns) and a distal portion (30-70 microns). The distribution densities of noradrenergic nerve fibers were measured by quantitative image analysis using the Interactive Bild-Analyse System (IBAS). The distribution densities of NA nerve fibers in both portions of the ophthalmic artery were significantly higher (p < 0.01) in SHRSP than that in WKY rats. The difference in the density of NA fibers of the ophthalmic arteries between SHRSP and WKY rats was 1.9 times in the proximal portion and 1.5 times in the distal portion. The vascular smooth muscle cells of the ophthalmic arteries in SHRSP were observed by scanning electron microscope to examine the trophic effect of NA nerve fibers on the vascular smooth muscle cells. The smooth muscle cells of both portions of the ophthalmic arteries in SHRSP showed a smooth surface texture and no necrosis, and were very similar to those of WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro perfusion studies of resistance artery function in genetic hypertension

To examine the function of resistance-sized arteries in hypertension under in vitro conditions that approximate in vivo conditions as much as possible, we mounted segments of second-order mesenteric resistance arteries from spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive control rats aged 12 to 13 weeks in a perfusion myograph and exposed them to conditions of constant flow and pressure. The endothelial integrity was validated both functionally and histologically. Vascular sensitivity to norepinephrine was examined when the hormone was applied either intraluminally or extraluminally and before and after removal of the endothelium. Both endothelium-dependent and -independent dilatation was assessed by the intraluminal application of acetylcholine and sodium nitroprusside, respectively. Sodium nitroprusside was applied to arteries after endothelium removal. Arterial responses were measured by changes in intraluminal diameter recorded with a video camera and imaging system. Vessels from SHR demonstrated depressed endothelium-dependent relaxation but similar endothelium-independent relaxation and greater sensitivity to norepinephrine with both intraluminal and extraluminal application. Removal of the endothelium abolished the differences in sensitivity to norepinephrine between the two strains. The results demonstrate that resistance arteries from SHR when examined under in vitro perfusion display enhanced sensitivity to norepinephrine due to depressed endothelium-dependent dilatation, and the data suggest that functional modifications in the endothelium may play an important role in hypertensive vascular disease.

Increased density of fluorescent adrenergic fibers around the middle cerebral arteries of stroke-prone spontaneously hypertensive rats

The distribution of fluorescent adrenergic nerve fibers in the proximal portion (horizontal segment, Hs) and the three distal portions (major branches) of the middle cerebral arteries (MCA) was examined in stroke-prone spontaneously hypertensive rats (SHRSP) aged 10, 30, 60, 90, and 180 days, by the glyoxylic acid method. The results were compared with those in age-matched normotensive Wistar Kyoto (WKY) rats. While the distribution pattern of fluorescent nerve fibers in the proximal portion of WKY rats changed from a straight linear arrangement at 10 and 30 days of age to a network-like arrangement after 60 days, those from SHRSP showed a constant meshwork pattern throughout the entire examination period. In the distal portions of the MCA of both SHRSP and WKY rats at all ages examined, fluorescent nerve fibers formed a coarse network. The distribution densities of adrenergic nerve fibers in the proximal and distal portions of the MCA of SHRSP were significantly higher (P less than 0.01 and 0.05) than those of WKY rats at all ages examined, except in the proximal portion at 90 and 180 days of age. The difference in nerve fiber density between SHRSP and WKY rats reached a peak at 30 days of age in both proximal and distal portions, and then gradually decreased with age. The present study suggests that sympathetic hyperinnervation is an important factor in the development of hypertension, and is involved in its maintenance in SHRSP.

The microvasculature in skeletal muscle. VI. Adrenergic innervation of arterioles in normotensive and spontaneously hypertensive rats

A microanatomical study of the adrenergic nerve plexus on the arterioles in the spinotrapezius muscle of normotensive and spontaneously hypertensive rats was carried out. The spinotrapezius muscle was selected since its microvasculature has been reconstructed in previous studies of this series. A modified glyoxylic acid amine densification technique was used to visualize the major portion of the microvascular nerve plexus. The nerve plexus density was quantified in the form of fiber length per unit area of vascular smooth muscle media. The adrenergic innervation was found to be limited to the arterial/arteriolar side of the microcirculation and positioned in close vicinity to vascular smooth muscle, in line with previous reports. Substantial variations of the nerve plexus density could be detected along the arterioles. Arcade arterioles show a significant reduction of the adrenergic innervation compared to that of the thoracodorsal supply artery. There was a significant elevation of the nerve plexus density at the origin of the transverse arterioles at the arcade arterioles, a site that in the past has been shown to exhibit the highest microvascular tonus in all arterioles of this organ. Distal to this site, transverse arterioles exhibit a progressive reduction of adrenergic plexus density toward their capillary endings, in line with the termination of vascular smooth muscle in these small branches. Sporadic fiber extensions were encountered leading from some of the transverse arterioles into the capillary network per se, but no regular innervation was detected in capillaries or in venules. These results suggest that the transverse arterioles may play a central role in nervous control of blood flow to the capillaries of muscle. Compared with the Wistar and Wistar-Kyoto strain, the spontaneously hypertensive rats exhibit qualitatively a similar pattern, but show quantitatively a significantly higher plexus density in the thoracodorsal artery and the arcade arterioles, a factor that may contribute to the elevated arteriolar tone.

Arterial neuroeffector responses in early and mature spontaneously hypertensive rats

Intramural sympathetic neuroeffector responses and presynaptic regulation of neurotransmission by amine uptake and alpha 2-adrenergic receptors were examined in young (5-week-old) and mature (12-week-old) spontaneously hypertensive rats (SHR) and were compared with those of age-matched Wistar-Kyoto (WKY) control rats. Electrical field stimulation (20 V, 0.2-msec pulse width, 3-second pulse train each minute, 5-100 Hz) elicited contractile responses from isolated mesenteric arteries mounted in a myograph. There was a significant difference between the sensitivity of arteries to electrical field stimulation in the two age groups, with arteries from 12-week-old rats being more sensitive than arteries from 5-week-old animals. Also, there was a significant age-strain interaction: the sensitivity of arteries from SHR to electrical field stimulation increased dramatically with age compared with that of WKY rat arteries. Cocaine significantly increased the sensitivity to electrical field stimulation after inhibition of presynaptic alpha 2-adrenergic receptors, and had a significantly greater effect in arteries from 5-week-old SHR compared with WKY controls. This would reflect an overactive neuronal amine uptake mechanism in young SHR. At 12 weeks there was no significant interstrain difference in the effect of cocaine. Yohimbine increased the sensitivity to electrical field stimulation both before and after inhibition of neuronal amine uptake, but there was no difference in its effect with age or strain. Therefore, although sensitivity to sympathetic nerve stimulation varies with age in the SHR, there is no evidence that this can be ascribed to alpha 2-adrenergic receptor function.

New role of nerve growth factor--an inhibitory neuromodulator of adrenergic transmission

In the present study, we investigated the effects of nerve growth factor (NGF) on norepinephrine (NE) release from peripheral sympathetic nerve endings of rat mesenteric artery. We made isolated mesenteric artery-intestinal loop preparations, by the modified method of Castelluci et al., from 4- and 8-week-old Wistar rats. NGF produced a dose-dependent inhibition of NE overflow from sympathetic nerve endings evoked by electrical nerve stimulation in the range of 0.1-10 ng/ml. Inhibition of NE overflow also occurred in the presence of a neuronal uptake blocker, desipramine (5 x 10(-8) M). NGF showed no effect on pressor response to exogenous NE (1 micrograms). These results suggest that NGF inhibits NE release from sympathetic nerve endings, in other words, NGF acts as an inhibitory neuromodulator of adrenergic transmission. This function of NGF might be considered as an inhibitory feedback mechanism against catecholamine-stimulated NGF synthesis.

Hyperreactivity of alpha 1-adrenoceptors, but not of P2X-purinoceptors, in vas deferens of spontaneously hypertensive rats

We evaluated the contractile reactivity to various stimuli, and the content and release of noradrenaline (NA) from a non-vascular tissue, the vas deferens, isolated from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The concentration-contraction curves for NA in tissue from animals of two ages (10-25 weeks and 30-45 weeks) were shifted to the left in SHR as compared with in age-matched WKY, with significant differences at 1.0 and/or 10 microM of NA. Similarly, the amplitude of contraction produced by electrical stimulation at 4, 8 and 16 Hz in the tissue was much larger in SHR than in WKY. However, ATP (10-100 microM) evoked contractions of the tissue to a similar extent in both SHR and WKY. The electrically evoked contractions of vas deferens from both strains were inhibited by isoprenaline in an approximate dose-dependent and equipotent manner. The tissue NA content, determined by HPLC-ECD, was nearly same in both SHR and WKY. In addition, the same amount of NA was released from the vas deferens of both strains by electrical stimulation in the presence of 4-aminopyridine. The present findings indicate that the contractile response of vas deferens to stimulation of alpha 1-adrenoceptors, but not of beta-adrenoceptors or P2X-purinoceptors, is more pronounced in SHR than in WKY and that a response indicative of hypertension may also occur in non-vascular tissue as it does in vascular tissue.

Pathogenesis of the essential hypertensions

The pathogenesis of essential hypertension (EH) is reviewed with a special focus on the development phase or the pre-hypertensive period. Three animal models are presented: the spontaneously hypertensive rat, the Dahl's salt-sensitive rat, and the Milan hypertensive rat. Some of the findings in animal models have inspired new fields and technical approaches for studying EH in man. From the original idea of Page, a new mosaic of various etiological parameters serves as a basis for reviewing the multiple facets of EH in man. One must conclude that EH is heterogeneous disease and most likely every single hypertensive patient belongs to a subgroup of the whole population of hypertensives.

Presynaptic regulation of neurotransmitter release in hypertension

1. The present review discusses evidence in support of the concept that alterations in sympathetic neurotransmitter release might contribute to the pathogenesis of hypertension. 2. Studies suggest that changes in sympathetic nervous activity in both the central and peripheral nervous systems participate in blood pressure control. 3. In the periphery increased release of norepinephrine from vascular adrenergic neurons might lead to the enhanced vasoconstrictor responses and thus to an elevation in systemic blood pressure. 4. The amount of neurotransmitter release from sympathetic nerve endings can be regulated by autoregulatory systems by presynaptic receptors located on nerve terminals. 5. It has been proposed that alterations to sympathetic nervous activity of hypertension might be partially due to abnormalities in presynaptic modulation of neurotransmitter release in central and peripheral tissues. 6. This article summarizes the results of studies to evaluate presynaptic receptor functions and sympathetic neurotransmitter release in hypertension.